Re: [PATCH v2 02/12] mfd: stm32-fmc2: add STM32 FMC2 controller driver

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 24 Apr 2020 13:47:34 +0200
Christophe Kerello <christophe.kerello@st.com> wrote:

> On 4/24/20 1:06 PM, Marek Vasut wrote:
> > On 4/24/20 12:50 PM, Lee Jones wrote:  
> >> On Fri, 24 Apr 2020, Marek Vasut wrote:
> >>  
> >>> On 4/24/20 9:45 AM, Lee Jones wrote:  
> >>>> On Wed, 15 Apr 2020, Christophe Kerello wrote:
> >>>>  
> >>>>> The driver adds the support for the STMicroelectronics FMC2 controller
> >>>>> found on STM32MP SOCs.
> >>>>>
> >>>>> The FMC2 functional block makes the interface with: synchronous and
> >>>>> asynchronous static memories (such as PSNOR, PSRAM or other
> >>>>> memory-mapped peripherals) and NAND flash memories.
> >>>>>
> >>>>> Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
> >>>>> ---
> >>>>> Changes in v2:
> >>>>>   - remove ops from stm32_fmc2 structure
> >>>>>   - add 2 APIs to manage FMC2 enable/disable
> >>>>>   - add 2 APIs to manage FMC2 NWAIT shared signal
> >>>>>
> >>>>>   drivers/mfd/Kconfig            |  12 +++
> >>>>>   drivers/mfd/Makefile           |   1 +
> >>>>>   drivers/mfd/stm32-fmc2.c       | 136 +++++++++++++++++++++++++
> >>>>>   include/linux/mfd/stm32-fmc2.h | 225 +++++++++++++++++++++++++++++++++++++++++
> >>>>>   4 files changed, 374 insertions(+)
> >>>>>   create mode 100644 drivers/mfd/stm32-fmc2.c
> >>>>>   create mode 100644 include/linux/mfd/stm32-fmc2.h
> >>>>>
> >>>>> diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
> >>>>> index 2b20329..5260582 100644
> >>>>> --- a/drivers/mfd/Kconfig
> >>>>> +++ b/drivers/mfd/Kconfig
> >>>>> @@ -1922,6 +1922,18 @@ config MFD_ROHM_BD71828
> >>>>>   	  Also included is a Coulomb counter, a real-time clock (RTC), and
> >>>>>   	  a 32.768 kHz clock gate.
> >>>>>   
> >>>>> +config MFD_STM32_FMC2
> >>>>> +	tristate "Support for FMC2 controllers on STM32MP SoCs"
> >>>>> +	depends on MACH_STM32MP157 || COMPILE_TEST
> >>>>> +	select MFD_CORE
> >>>>> +	select REGMAP
> >>>>> +	select REGMAP_MMIO
> >>>>> +	help
> >>>>> +	  Select this option to enable STM32 FMC2 driver used for FMC2 External
> >>>>> +	  Bus Interface controller and FMC2 NAND flash controller. This driver
> >>>>> +	  provides core support for the STM32 FMC2 controllers, in order to use
> >>>>> +	  the actual functionality of the device other drivers must be enabled.  
> >>>>
> >>>> Not sure how many times I have to say this before people stop
> >>>> attempting to pass these kinds of relationships off as MFDs:
> >>>>
> >>>> A memory device and its bus is not an MFD.  In a similar vain to the
> >>>> thousands of USB, I2C, SPI, PCI and the like devices that aren't MFDs
> >>>> either.
> >>>>
> >>>> Please find another way to associate your device with its bus.  
> >>>
> >>> This FMC2 is however an IP which can either operate external devices
> >>> (like ethernet chip on this parallel bus) or external flashes (like NOR
> >>> and NAND chips).  
> >>
> >> I'm sure that it *can*.  Although that's not its main purpose.  
> > 
> > I use it to operate KSZ8851-16MLL ethernet chip, which has async bus
> > interface. Linux just didn't have support for that mode of operation
> > thus far and the FMC was used to operate NANDs and NORs only. This
> > series, or rather, the first three patches in this series, add support
> > for operating other bus devices, like this ethernet controller.
> >   
> >> The
> >> clue is in the nomenclature ("Flexible *Memory* Controller").  Nor is
> >> it how the device is being used in this submission:
> >>
> >>    "The FMC2 functional block makes the interface with: synchronous and
> >>     asynchronous static memories (such as PSNOR, PSRAM or other
> >>     memory-mapped peripherals) and NAND flash memories."
> >>
> >> As I mentioned, this is just another memory device and its bus.  
> > 
> > I don't think it's _just_ a memory controller, it's more universal than
> > that, see above. Note that SRAM interface basically boils down to
> > anything which has external parallel bus, e.g. Davicom DM9000, that
> > KSZ8851-16MLL etc.
> >   
> >>> Can you provide a suggestion how this should be handled, if not as MFD?
> >>> It seems to me, that this is a Multi-Function Device .  
> >>
> >> Simply move it into the MTD or Memory subsystems and set up the
> >> dependencies via Kconfig.
> >>  
> >>> If this discussion is a recurring topic, is there some documentation
> >>> which explains how such devices should be handled ?  
> >>
> >> Not that I'm aware of.  
> > 
> > I see.
> >  
> 
> Hi Lee, Marek,
> 
> I will move this source code in the FMC2 bus driver. I think that I 
> should be able to manage the 2 controllers with 2 drivers (the FMC2 bus 
> driver and the FMC2 raw NAND driver).

FWIW, that's what I did for the Atmel EBI (External Bus Interface)
controller (see [1]).

[1]https://elixir.bootlin.com/linux/v5.6/source/drivers/memory/atmel-ebi.c